down fill power
The warmth of down depends on the size of the air pockets inside it. Down's warmth relative to its weight is measured as fill power.
Fill power is a measure of the volume created by a specific weight of down. The higher the fill power, the warmer the same weight of down.
US/IDFB is often used to measure fill power.
When performing measurement tests, the down is first stored and conditioned to bring it to the proper conditions for the test. This will ensure that the down is fully inflated.
30g of conditioned down is placed in a cylinder with a base area of 1 square inch.
Then slowly lower the cylinder lid onto the down to find a stable height.
Typically, three tests are performed and the average value is used as the final fill power.